preparation

Stir mustard seeds in dry skillet over medium heat until lightly toasted and starting to pop, about 3 minutes. Transfer to bowl; cool. Add vinegar, mustard, and shallots; whisk to blend. Gradually whisk in oil. Season with salt and pepper.

Toss watercress in large bowl with enough dressing to coat lightly. Divide watercress among plates. Combine celery root and radishes in same bowl; toss with enough of remaining dressing to coat. Season to taste with salt and pepper. Top watercress with celery root mixture and serve.